DNA Adduct Formation from Metabolic 5′-Hydroxylation of the Tobacco-Specific Carcinogen N′-Nitrosonornicotine in Human Enzyme Systems and in Rats

N′-Nitrosonornicotine (NNN) is carcinogenic in multiple animal models and has been evaluated as a human carcinogen. NNN can be metabolized by cytochrome P450s through two activation pathways: 2′-hydroxylation and 5′-hydroxylation. While most previous studies have focused on 2′-hydroxylation in target tissues of rats, available evidence suggests that 5′-hydroxylation is a major activation pathway in human enzyme systems, in nonhuman primates, and in target tissues of some other rodent carcinogenicity models. In the study reported here, we investigated DNA damage resulting from NNN 5′-hydroxylation by quantifying the adduct 2-(2-(3-pyridyl)-N-pyrrolidinyl)-2′-deoxyinosine (py-py-dI). In rats treated with NNN in the drinking water (7-500 ppm), py-py-dI was the major DNA adduct resulting from 5′-hydroxylation of NNN in vivo. Levels of py-py-dI in the lung and nasal cavity were the highest, consistent with the tissue distribution of CYP2A3. In rats treated with (S)-NNN or (R)-NNN, the ratios of formation of (R)-py-py-dI to (S)-py-py-dI were not the expected mirror image, suggesting that there may be a carrier for one of the unstable intermediates formed upon 5′-hydroxylation of NNN. Rat hepatocytes treated with (S)- or (R)-NNN or (2′S)- or (2′R)-5′-acetoxyNNN exhibited a pattern of adduct formation similar to that of live rats. In vitro studies with human liver S9 fraction or human hepatocytes incubated with NNN (2-500 μM) demonstrated that py-py-dI formation was greater than the formation of pyridyloxobutyl-DNA adducts resulting from 2′-hydroxylation of NNN. (S)-NNN formed more total py-py-dI adducts than (R)-NNN in human liver enzyme systems, which is consistent with the critical role of CYP2A6 in the 5′-hydroxylation of NNN in human liver. The results of this study demonstrate that the major DNA adduct resulting from NNN metabolism by human enzymes is py-py-dI and provide potentially important new insights into the metabolic activation of NNN in rodents and humans.